Methods allowing the systematic profiling of protein expression in mammalian cells will have a major impact in elucidating the role(s) of such proteins and may ultimately provide new targets for therapeutic development. The investigators propose to design novel tools for the discovery and functional analysis of proteins in mammalian cells based on protein trapping. The protein trap system consists of a mobile genetic element (protein trap vector) harboring an artificial exon encoding a protein tag. Upon insertion of the protein trap vector within an intron of a cellular gene, protein tag sequences will be conserved in the mature mRNA. Translation will result in a fusion of the protein tag to both the amino- and carboxyl-terminal parts of the trapped protein. Ideally, the tagged protein will retain the localization properties and functions(s)of the wild-type protein. This approach will facilitate gene expression analyses at the protein level, and can be used to gain specific empirical information about the potential biological functions of such proteins. Cell lines harboring tagged proteins will also lend themselves to study the temporal an spatial distribution of such proteins in response to specific drugs and during cellular differentiation and development.